1. Field of the Invention
The present invention relates to light beam scanning apparatus capable of preventing distortion in an image recorded with a light beam.
2. Description of the Prior Art
In recording apparatus, such as a laser beam printer, facsimile etc., utilizing conventional light beam scanning apparatus (hereinafter referred to as scanner), a rotational angle of the scanner usually changes sinusoidally in time at a determined frequency in consideration of its drive efficiency and of achieving high-speed drive. Also in order to prevent a distortion in an image recorded with a light beam, the maximum angle of rotation of the scanner is required to be constant throughout the scanning lines. This has conventionally been achieved by the technique to be discussed with reference to FIG. 1, wherein there are schematically shown a sinusoidal oscillator 1 generating a signal of a frequency of vibration of the scanner and composed of a known oscillator such as an LC oscillator, CR oscillator, crystal oscillator, frequency synthesizer etc., an automatic gain control 2 (hereinafter referred to as AGC circuit), for example, composed of an attenuator consisting of a combination of a known lamp and a CdS photocell or an attenuator utilizing an FET, a power amplifier circuit 3, a scanner driver 5 having an input terminal 4 thereof, a mirror 6 for reflecting the light beam, and a terminal 7 for releasing information on an angle or rotation of the scanner.
The output of sinusoidal oscillator 1 is supplied through the AGC circuit 2, power-amplified through the power amplifier circuit 3 and drives the scanner driver 5 through the input terminal 4. In driver 5 there is provided a generator coaxial with the rotating shaft of the mirror 6 and consisting of a coil and a magnetic circuit to generate a voltage corresponding to a rotational anuglar velocity of the mirror 6, the voltage being transmitted to the AGC circuit 2 through the output terminal 7. The AGC circuit 2 functions to decrease or increase the output thereof respectively when the rotating angle of mirror 6 is excessively large or small thereby maintaining the angle of rotation of the scanner constant. In the above-mentioned method, however, due to the use of the principle of a generator for the measurement of amount of rotation wherein the output voltage V of the generator is associated with the magnetic flux .phi. in the coil by the following equation: EQU V.varies.d.phi./dt . . . (1)
the output voltage V includes information on the rotational angular velocity of mirror 6 and not information on the angle of rotation thereof. Also stability of the voltage induced across the coil can hardly be guaranteed due to possible changes in the magnetic flux of the magnet constituting the generator or in the positional relationship thereof with respect to the coil, those changes being time- or temperature-dependent. Consequently the above-explained conventional method is insufficient for maintaining an exact angle of rotation.